Retirement Plans - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Reduction of defined benefit plan liability				$ 426.5
Reduction of defined deferred tax assets				351.3
Reduction of defined accumulated other comprehensive loss				906.1
Pension liability, before tax			$ 5.8
Pension liability, after tax			3.6
Reduction in pension obligation				354.8
Pension and postretirement contributions		$ 17.9	16.4	22.5
Defined benefit plan, accumulated benefit obligation	$ 921.9	$ 921.9	1,032.0
Threshold for recognition in net periodic benefit costs, percentage of projected benefit obligation or fair value of plan assets		10.00%
Multi-Employer Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Total contributions		$ 6.6	6.6
Multi-employer pension plan withdrawal obligations
Defined Benefit Plan Disclosure [Line Items]
Other Charges		$ 3.0	3.0	3.0
Hedging Investments
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage	50.00%	50.00%
Return Seeking Securities
Defined Benefit Plan Disclosure [Line Items]
Target asset allocation percentage	50.00%	50.00%
Net Investment and Other Income
Defined Benefit Plan Disclosure [Line Items]
Pension and other post retirement non-cash settlement expense				21.1
Net Earnings from Discontinued Operations
Defined Benefit Plan Disclosure [Line Items]
Pension and other post retirement non-cash settlement expense				77.7
Lump Sum Pension Payment Or Annuity
Defined Benefit Plan Disclosure [Line Items]
Benefits Paid				328.4
Pension Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Reduction in pension obligation		$ (0.7)	0.0
Benefits Paid		45.2	44.6
Pension and postretirement contributions		10.7
Medicare reimbursements		0.0	0.0
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Reduction in pension obligation		32.6	0.0
Benefits Paid		33.9	34.8
Pension and postretirement contributions		7.2
Reduction in pension liability due to curtailment				35.0
Medicare reimbursements		6.4	$ 5.0
Other Postretirement Benefit Plans, Defined Benefit | Canada
Defined Benefit Plan Disclosure [Line Items]
OPEB retiree contribution term	3 years
OPEB expiration year	2021
Adjustments in OPEB liability	$ 31.1
Other Postretirement Benefit Plans, Defined Benefit | Net Investment and Other Income
Defined Benefit Plan Disclosure [Line Items]
Curtailment gain				$ 19.5
Pension and Retirement Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement expected contributions for next year	$ 8.0	8.0
EGWP
Defined Benefit Plan Disclosure [Line Items]
Medicare reimbursements		$ 6.4